Meltblown ionomer microfibers and non-woven webs made therefrom for gas filters

ABSTRACT

Ethylene/carboxylic acid ionomers may be made, using a meltspun, and particularly meltblown process, into microfibers and thence to filter webs which are efficient gas filters for removing particles having a diameter of from 0.5 to 20 microns from a gas. The webs are effective without a deliberate specific post-charging operation during production. A deliberate post-charging operation can also be carried out to result in further efficiency.

This is a division of application Ser. No. 08/712,743, filed Sep. 12,1996, now allowed, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to microfibers made from ethylene/carboxylic acidionomers, the fibers being meltspun, particularly using the `meltblown`process. The microfibers in the form of a web material can be efficientgas filters without being electretized. The materials, however, may alsobe electretized.

2. Description of Related Art

Non-woven materials are well known and widely used in a variety ofapplications including apparel, adhesives, sorbents and filters. Thesematerials are made from matted, entangled non-bonded, but alsomelt-bonded fibers. When the matting is very tight, the non-wovenmaterial may be thought of as fabric-like. Such non-wovens may beuseful, for instance, for apparel. When the matting is relatively looseand open it may considered, and is often referred to as, a `web`. Theform of a web may be thick or thin and the fibers entangled and/orbonded with varying degrees of openness, and with variations in thedegree of bonding.

The properties of non-woven materials vary widely, from tough torelatively weak, flexible to stiff, highly porous to having lowporosity, with highly absorptive or less absorptive capacity, yet mayeven have barrier characteristics, especially to liquids. The nature ofnon-wovens depends on (i) what material is used to make the fibers (ii)the nature of the fibers which will depend on the process used to makethem, and (iii) how the fibers are bonded together. It may be fair tosay that the variation in the type and applications of non-wovens is asgreat or greater than that of woven materials.

A major market for non-wovens is in the filtration market, for gases andliquids. Gas filtration involves removing particulate matter, usuallysolid such as dust, but also liquid particles, from a gas, particularlyair. Typical markets include heating ventilating and air conditioning(HVAC). Demanding markets such as pharmaceuticals, microelectronics andbiotechnology use highly efficient or ultra efficient particulate airfilters (HEPA and UFPA).

Because of the differing gas filter applications, the demands of, andcharacteristics of gas filters are varied. The filter may require highflux (i.e., high gas throughput) requiring high permeability, and/orrequire a high level of particle removal, and/or require removal ofspecific size particles. To a large extent flux, often quantified andmeasured in terms of the `pressure drop` across the web, and the`filtration efficiency` are conjugate quantities. Thus tighter filterwebs may be more efficient but have lower permeability. The filtrationefficiency however must always be characterized in relation to theparticle size filtered. The drive in filter technology is towardsimproving the efficiency at given flux, or improving flux at a givenefficiency as well as reducing cost, of course.

The `architecture` of a filter web depends on the fiber diameters, andthe distribution of the diameters, how the fibers are entangled/bondedtogether, the density of web, its uniformity, and the thickness andweight per unit area. These architectural features are a major factor infilter efficiency, the flux, and the size particles which they remove.To filter out small particles, it is necessary to have fine fibers andsmall passages throughout the web, with no large channels (somewhatcomparable to a finer mesh in a woven filter). Webs made of`microfibers` as opposed to fibers of the size of normal textile fibersare used for filtering fine particles in the 0.1 to about 20 micronregion.

While fibers for filter webs are often made of glass, syntheticthermoplastic polymers are also commonly used. Microfibers of syntheticthermoplastic polymers are commonly meltspun, particularly by a processknown as the `meltblown` process, though certain other processes canproduce microfibers such as fibrillating film. The meltblown process hasbeen succinctly defined as `a one-step process in which high-velocityair blows a molten thermoplastic resin from an extruder die tip, onto aconveyor or take-up screen, to form a fine-fibered self-bonding web`. Ameltblowing apparatus suitable for the production of microfibers wasdescribed in Report No. 4364 of the Naval Research Laboratories,published May 25, 1954, entitled `Manufacture of Super Fine OrganicFibers`, by Van Wente et al.

The ability to make microfibers from synthetic polymers, and the natureof the resulting web produced in the meltblown process depends on themelt rheological and crystallizing or, more generally, solidifyingcharacteristics of the polymer. Other technologies that, in a broadersense, could be considered meltblown, or more generally meltspunprocesses, and can produce microfibers include electrostaticmelt-spinning, flash spinning and centrifugal spinning.

For filtering fine particles however, the appropriate web architectureis a necessary factor but may not be the only factor in determiningefficiency. Another major factor is the electrostatic nature of thesurface of the polymer fibers. This is significantly dependent on thechemical nature of the polymer composition, the molecular conformationwithin the fiber, and the surface nature of the fibers made from it.Generally, filter microfibers are subjected to a surface treatment toincrease their electrostatic charge or polar nature. So-called`insulating` polymers, those with high resistivity, are `electretized`which is sometimes said to mean `electrified to possess permanentdielectric polarization` or `electrified to make an electret` or topossess an `electret surface`.

A deliberately produced electret surface on filter web microfibers maybe produced at different stages of forming the filter. A material may betreated even before fiber formation, such as a sheet beforefibrillation. Fibers may be treated during or after their formation, orthe treatment may be carried out during or after the actual webformation Such treatment is conventionally done by a procedure involvingrubbing or corona charge treatment. Other techniques for providingelectret enhancement are described in U.S. Pat. No. 4,375,718(Wadsworth); No. 4,588,537 (Klaase) and No. 4,592,815 (Nakao).

U.S. Pat. No. 4,215,682 (Kubik et al.) teaches that filtering efficiencyof a meltblown microfiber web can be improved by a factor of two or morewhen made into an electret.

While electrets are often referred to as having a `permanent dielectricpolarization`, which leads to or is associated with a surface charge, infact to a greater or lesser degree, the electrostatic charge orpermanence of surface polar nature, whatever its precise nature, decayswith time. The usefulness of electret enhanced filtering is of coursedependent on how permanent the electret nature is, in relation to thetime span for use of the filter.

The most commonly used polymer for making such electret filters ispolypropylene but other fibers have been used. U.S. Pat. No. 5,411,576(Jones et al.) states that other polymers may be used, such aspolycarbonates and polyhalocarbons, that may be meltblown and haveappropriate volume resistivities under expected environmentalconditions.

U.S. Pat. No. 4,626,263 (Inoue) discusses the characteristics ofelectets made of non-polar and polar polymers. Non-polar polymers suchas polyethylene and polypropylene are indicated to produce stableelectrets, but of relatively low electrostatic charging capacity, whilepolar polymers are indicated to have high initial electet capacity, butrelatively rapid decay, particularly under humid conditions.

U.S. Pat. No. 4,789,504 (Ohmori et al.) discloses microfibers which areable to be more permanently electretized than prior art materials. Thematerials used to make the fibers consist of polymers which includepolypropylene, polyethylene, polyester, polyamide, poly(vinyl chloride),poly(methyl methacrylate) etc., containing 100 ppm or more, preferably200 to 2000 ppm, in terms of the metal, of a fatty acid salt such as analuminum, magnesium or zinc salts of palmitic, stearic or oleic acid.

Electretizing generally requires extra steps and use of specialequipment after or during making of the fibers or webs. There is a needfor materials which can be made into filter webs without the need topost-charge or deliberately electretize, yet which have comparable orbetter efficiency/flux. At the same time, if such materials can beelectretized to produce filters with even greater efficiency, then thematerials serve yet an added need.

SUMMARY OF THE INVENTION

The invention depends on the recognition that ethylene/carboxylic acidionomers appear to be uniquely suitable for microfibers for filters,particularly since it is not necessary to deliberately post-charge orelectretize the fibers or fiber webs, though they may additionallybenefit from a deliberate electretizing process. The fibers have highstatic retention of any existing or deliberate specifically inducedstatic charge which makes them excellent gas filters either withoutelectretizing, as well as with an electretizing step.

Specifically, the invention comprises meltspun, particularly meltblownmicrofibers for filter webs the fibers made from a copolymer comprising:

a copolymer of ethylene, 5 to 25 weight percent of (meth)acrylic acid,and optionally, though less preferably, up to 40 weight percent of analkyl (meth)acrylate whose alkyl groups have from 1 to 8 carbon atoms,having from 5 to 70 percent of the acid groups neutralized with a metalion, particularly zinc, sodium, lithium or magnesium ions, or mixturesof these, the copolymer having a melt index of from 5 to 1000 g/10minutes.

A further aspect of the invention are non-electretized filter webs madefrom the above fibers. Yet another aspect are electretized filter websmade from the above fibers. Yet a further aspect of the invention aremicrofibers and webs of those microfibers where the material used tomake the fibers is a blend of the above ionomer and another polymer,where the ionomer is at least 10 weight percent of the blendcomposition.

DETAILED DESCRIPTION OF THE INVENTION

The precise nature of an `electret surface` is variable and not alwayssubject to easy characterization. While an electret surface is normallyinduced by various deliberate treatments, the polymers of this inventionmay possess an electret-like surface without deliberate post-chargetreatment of the fibers or web. Therefore, in view of certain ambiguityto the term electret, the claims simply distinguish between materialwhich has been `deliberately electretized` by the various disclosedtreatments and those that have not. The question as to whethernon-treated surfaces of the polymers of this invention have a surfacewhich could be described or resemble an `electret surface` or is anelectret is thus avoided.

In this disclosure, the word copolymer means a `direct` near randomcopolymer, (as distinct from a graft copolymer) polymerized from two ormore comonomers, and thus includes dipolymers, terpolymers etc.

Microfibers, as the term is used in this application means fibers havingan average diameter of less than about 30 microns.

The copolymers used to make the microfibers of this invention areionomers. Many ionomers are well known commercially. The ionomers ofthis invention are metal ion neutralized copolymers of ethylene withacrylic or methacrylic acid or both. The option of either or both ofthese acids will be designated by the shorthand term `(meth)acrylicacid`. lonomers of this type are sold under the tradename of SURLYN®ionomer resins, by E.I. du Pont de Nemours and Company, as well as othercompanies. The ionomer copolymers of the invention contain 5 to 25weight percent, preferably 8 to 20 weight percent (meth)acrylic acid,most preferably 8 to 15 weight percent. Optionally they may contain upto 40 weight percent of a `softening` monomer which is an alkyl acrylatewith a 1 to 8 carbon alkyl group. Preferably however, the ionomer is adipolymer with no softening monomer. The carboxylic acid groups arepartially neutralized; from 5 to 70 percent, preferably 25 to 60 percentof the acid groups being neutralized with metal ions, preferably ofsodium, zinc, lithium or magnesium (or mixtures of these), mostpreferably sodium or zinc. Ions which produce more hydrophilic ionomerssuch as potassium are less preferred unless the degree of neutralizationis at a relatively low level. Non-neutralized acid copolymers are lesssuitable for making the microfibers of this invention.

Acid copolymers and their preparation are described in U.S. Pat. No.4,351,931 (Armitage), and ionomers and their preparation from acidcopolymers are described in U.S. Pat. No. 3,264,272 (Rees). `Soft`ionomers containing alkyl acrylates are described in U.S. Pat. No.4,690,981 (Statz). Preparation of acid copolymers containing higherlevels of acid are advantageously prepared by a modified method,referred to as `cosolvent technology` which is described in U.S. Pat.No. 5,028,674 (Hatch et al.). All four above patents are herebyincorporated by reference.

A meltblowing apparatus suitable for preparation of the microfibers ofthe present invention is described in the Naval Research LaboratoriesReport 4364, discussed in the Related Art Section. Further descriptionsof meltblowing apparatus also suitable for the present microfibers havebeen more recently given in the TAPPI Journal, 78, 185, 1995 by SanjivR. Malkan, and by L.C. Wadworth et al. in `Melt Blown Technology Today`,Miller Freeman, 1989. Typically, and as employed in the presentinvention, the apparatus consists of an extruder, a metering pump, a dieassembly and web formation and winding equipment. The distinct featureis the die assembly, consisting of a polymer feed distribution system, adie nosepiece and air manifolds. The air manifolds supply thehigh-velocity hot air at temperatures between about 400 and 600° F.through slots in the die nosepiece. As soon as the molten polymer isextruded from the die holes, the high-velocity hot air streams attenuate(thin/divide) the polymer streams to form microfibers. As the hot airstream containing the microfibers progresses towards a collector screen,it draws in a large amount of surrounding air that cools and solidifiesthe fibers. The solidified fibers subsequently get laid randomly ontothe collecting screen, forming a self-bonded non-woven web.

U.S. Pat. 3,825,380 (Harding et al.) describes a die with specific noseconfiguration, for production of non-woven mat. This patent is alsohereby incorporated by reference.

For ionomers of the present invention, water jets are desirable tofacilitate microfiber cooling, producing a more uniform web, withoutoverly fused bond areas.

The microfibers of this invention have an average diameter in the rangeof 0.5 to 30 microns, preferably 0.5 to 10 microns and most preferably 1to 5 microns. Fibers in the broad range, but preferably in the narrowrange are of a suitable diameter for removing particles in the 0.1 to 20micron size, the size of particle removed by the fiber webs of thisinvention. Non-woven filter webs of this invention, produced from themicrofibers, have a weight of between 0.5 and 10 oz./square yard,preferably between 0.7 and 3 oz./square yard. The web thickness shouldbe between 5 and 30 mils.

Within these limits considerable variation is possible. It is within theskill of the artisan to vary the architectural dimensions and featureswithin these limits and improve, and eventually optimize the filteringefficiency/flux for the desired utility.

The microfiber filter webs of this invention, may be used as part of afilter structure, and may be laminated with a layer of a different(generally spunbonded) web, or can be in the form of a three-layersandwich in which the meltspun, particularly meltblown microfiber webforms the core, while the different spunbonded web forms the outerlayer. Spunbonded non-woven webs and its fibers of the type useful forsuch sandwich structures, can be made by any convenient process, forexample by melt-spinning the polymer as described in U.S. Pat. Nos.3,821,062 (Henderson), 3,563,838 (Edward) and 3,338,992 (Kinney). Thedifferent spunbonded web fibers of the sandwich should have a diameterof at least 20 microns and thus is considerably higher than thepreferred microfiber diameter of 1 to 5 microns used for the core web.The different spunbonded layer provides mechanical strength andintegrity, while the core layer provides the desired microporosity andfilter properties.

The filter webs need not be deliberately electretized to be highlyefficient filters. That is not to say that the webs can not benefitfurther from a specific post-charging/electretizing step. In briefpreliminary experiments discussed below, on a less than optimum ionomerweb, post-charging/electretization was found to improve filteringefficiency, just as it is known to do for other microfibers.

In past tests prior to the present development study, rate of decay ofstatic charge of a SURLYN® ionomer resin which was an ethylenemethacrylic acid copolymer with 10 weight percent acid neutralized withzinc was compared with a typical LDPE. Static retention at 100 secondswas 96% for the SURLYN® ionomer resin compared with 55% for the LDPE,while at 1000 seconds it was 94% compared with 25% for the LDPE. Whilethe test was not carried out on a filter and the data can not betranslated to filter behavior precisely, it nevertheless suggests asignificant difference in behavior between ionomer resins andpolyethylene. Ionomers have high volume resistivity. It is notcompletely understood why, in view of the highly polar nature of ionomerresins, they should make such good filters, have such high resistivity,and retain static charge so well.

The ionomers of the invention can have a melt index (MI) anywherebetween about 5 and 1000 g/10 minutes. However, because a moderatelyfluid melt is desirable for meltblowing, the MI should preferably bebetween 30 and 400 g/10 minutes more preferably between 60 and 350 g/10minutes and most preferably between 50 and 350g/10 min. The more fluid,the easier it is to produce fine microfibers, but too high an MI maydetract from fiber strength, and may lead to excessive fiber bonding. Itis also important to have the polymer solidify with adequate bonding offibers, but not too much `coalescence` by excess fusing, which producesareas where the fibers lose their fibrous identity. For this reason,generally, the more rapid the crystallization and higher melting point,the better. Ionomers, have low softening temperatures, and in thisregard require special care in meltblowing a satisfactory web.Dipolymers are preferred to terpolymers because they crystallize morereadily. Other means which can raise melt temperature of the ionomer,such as polymerizing at a temperature lower than about 200° C. may helpalso. However, standard ionomers prepared at between 220° C. and 270° C.and pressures between 23,000 and 25,000 psi, according to the Reespatent noted above, are entirely suitable. Nevertheless, it has beenfound highly advantageous to cool the microfibers as quickly as possiblewith water, to prevent excess bonding.

Ionomer microfibers produce highly efficient filter webs. Thisefficiency may also be utilized in mixed fiber webs also containingfibers other than ionomers.

Ionomers whether deliberately post-charged or not, in the form of filmor sheets may also find applications as the electrostatic element inelectro-acoustical devices such as microphones, headphones and speakers,generators and recorders.

EXAMPLES

Microfiber webs of ionomer resins, (either experimental or commercialgrades), as well as webs of two grades of HDPE and LLDPE were prepared.Webs were also made of blends of ionomer resins and polypropylene andblends of ionomer resins and polyethylene.

The meltblown webs were made in a 20 inch meltblowing line consisting ofa 2 inch screw extruder and metering pump to control the polymerthroughput per die hole in the range of 0.3 to 2 grams/minute. The 20inch horizontal die had 25 holes per inch and 0.0145 inch diameterholes. A furnace for heating the air, and an air compressor with amaximum capacity of 650 sq.ft./minute was used to attenuate the fibersfrom the die-tip to a moving drum collector. Except for a first,preliminary series of scouting tests, high-pressure water jets wereintroduced 2 to 3 inches from the die to enhance fiber cooling for theionomer resin webs, before collecting on the drum. The die to collectordistance was maintained, for most webs, at 11 inches. Conditions to makewhat is believed to be close to optimum fiber diameter are shown inTable 1.

Pore size distribution (minimum, maximum and mean) of the web wasmeasured using a Coulter Porometer, as described in ASTM F-316-86.

The filtration efficiency was measured with polystyrene spheres ofnominal sizes of 0.6, 1, 2 and 3 microns in diameter, at the 30 cm/secface velocity. Pressure drop across the filter media is reported ininches of water. The test is a modified ASTM 1215 filter efficiency testin that four different particle sizes were tested simultaneously.

A first preliminary scouting series of tests utilized an ionomer resinfilter web made, as noted above, without water cooling. As suggestedabove, it is believed the low crystallization temperature of theionomers which is lower than the other polymers tested, produced a webwith fused spots, leading to less than optimum filtration efficiency.The results shown in the table as Test Series 1 should therefore not beconsidered representative of the real potential of ionomer microfiberwebs. After these first tests, water cooling of the web was employed,and this produced an ionomer filter web with much greater efficiency. Inthis test series the ionomer fibers had diameters in the range of 2 to 8microns, still somewhat higher than the goal diameter of 1 to 5 microns.Results of these tests carried out on the ionomers and polyethylenesamples, together with a commercial post-charged polypropylene web, anda commercial auto cabin filter based on polycarbonate resin are shown inTable 2.

As noted above, the filtration efficiency and flux, as indicated bypressure drop for a filter web depends on two principle factors: (i) theweb architecture and (ii) the polymer and its surface charge. Incomparing two polymers, the quite different melt and crystallizationcharacteristics of different polymers make it difficult to achieve trulycomparable webs of similar architecture for the two polymers. The onlytotally satisfactory comparison of filtering efficiency would be tocompare different polymers with the best possible web achievable with ofeach of those polymers. One would have to vary web making conditionsextensively, and test each, selecting the web which gave the bestresults for a given polymer. Unfortunately, one could never be sure thatthe best web for that polymer for a given filter requirement wasproduced. Microscopic examination however can give a crude guide as towhether two polymers had comparable architecture.

The difference in the results for ionomer in series 1 and 2, as shown inthe table, make it clear that the ionomer webs in the first series wereless than optimum webs, the second series producing far superiorresults. As indicated this is believed to be due to the low meltingpoint of the ionomer resin, producing fused webs.

The first series also included ionomer web which had been specificallydeliberately electrostatically post-charged for comparison withcommercial electretized polypropylene. In this test, filtrationefficiency for 0.1 micron particles, at generally comparable webthickness and pressure drop, was the same order for the ionomer resin asfor the polypropylene both for the charged and non-charged web samples,though somewhat higher for the polypropylene. Uncharged, the values were32 versus 41 for polypropylene and charged 87 versus 98 for thepolypropylene. LLDPE was very much poorer than either. While these datarelate to less than optimum webs, they indicate that ionomer resin, likepolypropylene resin, can benefit from deliberate post-charging. However,the actual values obtained, as indicated above, do not representsignificant numbers in terms of the efficiency possible with thematerial, in view of the non-optimum web structure.

The second series produced an ionomer web which gave far superiorresults to the first series ionomer webs. In this series of tests, whileit is impossible to be sure that each of the webs of different materialswas close to optimum for that material, the webs were generallymicroscopically comparable in appearance. The polycarbonate and thePP-electret were both commercial webs, and presumed to have beenproduced under reasonably optimum conditions. Web thickness is noted ifmeasured, but is not particularly critical, the critical criterion ofthe filter being efficiency as a function of pressure drop.

The results indicate that the ionomer which was not deliberately chargedis equal to or even superior to the commercial polypropylene electret,for particles from 0.6 to 3.0 microns diameter. The non post-chargedionomer sample with comparable web base weight of 2 oz./square yard issuperior to the charged polypropylene sample. In fact the polypropylenesample had a greater pressure drop (0.33 versus 0.23). Since efficiencyand pressure drop are largely conjugate characteristics of a filter, thenon-charged ionomer shows superior performance.

The conjugate nature of efficiency and pressure drop can be seen fromthe three ionomer samples. As the web base weight increases, so doesfilter efficiency, but at the expense of increased pressure drop from(0.1 to 0.23). The non-charged polycarbonate cabin filter is lessefficient than the ionomer at comparable pressure drop of 0.23.Comparing the polyethylenes, a comparison can be made at a pressure dropof about 0.1 (0.12 for LLDPE). When comparison is made at this pressuredrop, the ionomer is far superior to either the HDPE or the LLDPE.

The very high filtration efficiency of the ionomer resin for allparticles sizes, even without deliberate post-charging, makes it clearthat this is a truly unique and excellent material for microfiberfilters. It is quite possible that without a specific deliberatecharging operation, ionomer microfibers readily acquire a static chargeor an electret-like surface of some sort, which results in their highfilter efficiency with webs made from them.

A further series of tests was carried out on webs of blends of ionomerresin and either polypropylene or polyethylene. The two polymers werefed into the extruder and the extrudate blend microfibers made into aweb as usual. The results in Table 2 show that even 20 percent ionomerimproves the filter efficiency of blends either with polyethylene orpolypropylene. Increasing the amount of ionomer to 50% did not furtherimprove efficiency significantly. The ionomer presence dominates even atthe 20 percent level, and is particularly effective in the polyethyleneblends. The efficiency improvement is more marked for particles of 2 and3 microns than it is for particles of 1 micron and less. Based on theseresults, blends containing as little as about 10 percent ionomer arelikely to show at least some improvement over webs made from polyolefinalone.

                                      TABLE 1                                     __________________________________________________________________________    PROCESS CONDITIONS FOR MAKING WEBS                                            (For Series 2 and 3 as per Table 2)                                                 Throughput                                                                          Air flow rate                                                                       Air                                                               g/min. per                                                                          (cubic ft. per                                                                      Temp.                                                                             Die Temp.                                                                          Die Collector                                                                        Water Jet                                   Polymer                                                                             hole  minute)                                                                             °F.                                                                        °F.                                                                         distance (in.)                                                                       (On/Off)                                    __________________________________________________________________________    LLDPE 0.4   550   440 430  NM     Off                                         HDPE-1                                                                              0.4   610   443 430  NM     Off                                         HDPE-2                                                                              0.35  620   444 430  NM     Off                                         Ionomer                                                                             0.35  630   400 400  NM     On                                          Ionomer/PP                                                                    20/80 0.34  630   418 450  15     On                                          50/50 0.34  630   418 450  11     On                                          Ionomer/PE                                                                    20/80 0.34  610   400 400  11     On                                          50/50 0.34  610   400 400  11     On                                          __________________________________________________________________________     For description of polymers see Table 2. Conditions for the preliminary       scouting tests of series 1, table 2 were not recorded.                   

                                      TABLE 2                                     __________________________________________________________________________    FILTRATION EFFICIENCY OF VARIOUS MELTDOWN WEBS                                                  Web           Efficiency for particle                                                                      Pressure                       Test        Base Weight                                                                         Thickness                                                                          Pore Size (microns)                                                                    size: (microns)                                                                              Drop                           Series                                                                            Polymer (oz/sq. yard)                                                                       (mils)                                                                             Min.                                                                             Max                                                                              Mean                                                                             0.1                                                                              0.6                                                                              1.0                                                                              2.0                                                                              3.0                                                                              (in H.sub.2 O)                 __________________________________________________________________________    1   Ionomer 2.3   10.6          32             0.6                            1   Ionomer/Elec.                                                                         2.3   10.6          87                                            1   PP      2.1   9.3           41             0.7                            1   PP-Electret                                                                           2.1   9.3           98                                            1   LLDPE   1.7   10.7          11                                            2   LLDPE   1.1   6.9  15.7                                                                             97.3                                                                             32.6                                                                             16.2                                                                             22.4                                                                             23.3                                                                             39.8                                                                             67 0.12                           2   HDPE-1  0.8   7.9  18.3                                                                             157.8                                                                            37.2                                                                             10 7.3                                                                              11.3                                                                             28.2                                                                             52.9                                                                             0.1                            2   HDPE-2  0.8   8.0  19.1                                                                             208.6                                                                            40.6                                                                             7.5                                                                              4.5                                                                              5.8                                                                              19.4                                                                             39.2                                                                             0.08                           2   Ionomer 1.0   6.6  16.0                                                                             127.3                                                                            34.5                                                                             60 61 60.2                                                                             66.6                                                                             74.1                                                                             0.1                            2   Ionomer 1.4   10.4 17.9                                                                             93.2                                                                             33.0                                                                             77.3                                                                             73.7                                                                             71.8                                                                             82.2                                                                             88.2                                                                             0.18                           2   Ionomer 1.9   12.5 16.8                                                                             77 29.6                                                                             86.5                                                                             86 86.5                                                                             93.7                                                                             97.1                                                                             0.23                           2   Polycarbonate                                                                         7     NM               73.5                                                                             73.5                                                                             78 90.5                                                                             0.23                           2   PP-Electret                                                                           2     NM               83.2                                                                             84.8                                                                             92.3                                                                             95.3                                                                             0.33                           3   Ionomer/PP 20:80                                                                      2                      20 38 79 86 0.25                           3   Ionomer/PP 50:50                                                                      2                      27 43 80 89 0.25                           3   Ionomer/PE 20:80                                                                      2                      59 73 90 95 0.13                           3   Ionomer/PE 50:50                                                                      2                      59 73 91 96 0.14                           __________________________________________________________________________     Notes to Table 1 and 2:                                                       NM = not measured.                                                            LLDPE is Aspun ® 6831A mfg. by Dow Chemical, MI = 150                     HDPE1 is Alathon ® 5280 mfg. by Lyondell MI = 80                          HDPE2 is Alathon ® 5050 mfg. by Lyondell MI = 50                          The Ionomer is an E/MAA (90/10) by wt. ˜20% Zinc neutralized, MI =      300, an experimental ionomer polymer.                                         PPElectret, commercial product of 3M Company.                                 PP in Table 1 was a commercial nonelectretized web.                           PP used in blends, (series 3) was Exxon Grade PP1200                          Polycarbonate is a Cabin Filter mfg. by Frudenberg. Co.                       cfm = cubic feet/minute.                                                 

We claim:
 1. Microfibers, made from a polymer blend, consistingessentially ofa) at least 10 weight percent of a copolymer of ethylene,5 to 25 weight percent of (meth)acrylic acid, and optionally up to 40weight percent of an alkyl (meth)acrylate whose alkyl groups have from 1to 8 carbon atoms, having from 5 to 70 percent of the acid groupsneutralized with metal ions, the copolymer having a melt index of from 5to 1000 g/10 minutes b) the remainder being a polyolefin selected fromthe group consisting of polypropylene and polyethylene.
 2. Themicrofibers of claim 1 produced by a meltblown process.
 3. Themicrofibers of claim 2 wherein the ethylene (meth)acrylic acid copolymeris a dipolymer of ethylene and (meth)acrylic acid, having from 8 to 20weight percent acid, with from 25 to 60 percent of the carboxylic acidgroups neutralized with zinc, sodium, magnesium or lithium ions, or amixture of these ions, the dipolymer having a melt index of from 150 to350 g/10 minutes.